Kepler Focal Plane Array with each tile partly covered by something looking like a bar chart.

If real, what is it?

EDIT: I do not mean the seam between the two CCDs of each tile but the slighly curved stripes partly "covering" (if real) one CCD of each 2-CCD tile. The pattern occurs on the CCD that is closer to the camera and (EDIT 2) the stripes are oriented ~parallel to the seam between the two CCDs. Clearly not a reflection of roof structure as proposed by blobbymcblobby, Uwe, and uhoh.

enter image description here

  • $\begingroup$ I would have guessed it as object on roof that is reflecting. But clearly there are both vertical and horizontal "bar charts". Neither it can be refraction/reflective property it else each tile must exhibit it. But there is some symmetry on which side has the "bar chart". $\endgroup$
    – zephyr0110
    Jan 23 at 13:43
  • 4
    $\begingroup$ I'm pretty sure it is a reflection, though a distorted and broken-up one. Notice it appears on the "near" corner of every tile, and the bars both become clearer and have a sharper cut-off on the tiles tilted away, like it's some total internal reflection effect. The dark stripes might be where the structure of the sensors and whatever cover layers interferes with the reflection of the wall behind the array, or they might be multiple reflections smeared out into lines, etc. $\endgroup$ Jan 23 at 14:58
  • $\begingroup$ @ChristopherJamesHuff, "total internal reflection" is compelling. Please write it as an answer. Thanks to blobbymcblobby for the information about the covering lenses. You and Uwe are wrong about the reflection of roof structure. There ARE such reflections, some of which are spanning both CCDs of a module, e.g. module 13, but obviously unrelated to the "bar charts". $\endgroup$
    – Rainald62
    Jan 24 at 18:08

2 Answers 2


I think the bars are reflections from the clean room ceiling with many lamps and air condition outlets.

enter image description here enter image description here enter image description here

The reflections look different on each image.

The sensor array is not a flat plane to correct the curvature of the image surface.

We would need an image of the clean room ceiling for comparison, but the photographer concentrated on the sensor array.

  • $\begingroup$ I think part of the issue was that only one of each pair shows the reflections. In my answer I tried to show all the angles I could of the FPA showing the variations in reflections - from the lens - as the CCD is antireflective. I also suggested it was partially to do with alignment each pair (opposing) and the filter applied to the lens. There is another photo from the same session I found but the camera was obviously on a tripod and reflection issue was the same. I think the lens properties is mostly the issue but not the sole. $\endgroup$ Jan 24 at 18:44
  • $\begingroup$ The sensors of each pair seem to be mounted not in line but slightly tilted. If this is true, only one of each pair may show the reflections. $\endgroup$
    – Uwe
    Jan 24 at 19:08
  • $\begingroup$ Hard to tell if that's the case, I had it that each module (so each pair) is tilted to create the bowed FPA. The only take I could get is that they did not look for precision at mounting the actual CCD's or modules - a bolt and go - approach as the hard work would be done by the mirror, so that there was bound to be some variation in angles due to installation. $\endgroup$ Jan 24 at 19:13
  • $\begingroup$ The pairs of figure 11 show very similar but not identical reflections. $\endgroup$
    – Uwe
    Jan 24 at 19:13
  • $\begingroup$ Yes this is definitely the answer. There is progressive displacement of reflections of the same pattern from one to the next which matches the progressive change in direction of surface normal. It's always a challenge when photographing mirrors, curved or otherwise :-) $\endgroup$
    – uhoh
    Jan 25 at 23:48


So consider the image below:

The CCD chips have AR on them which, going by the graph, work up to a point, that being the angle of incidence at which the AR no longer works.

The green lines in the image roughly show the cut off point.

I found that the photograph was taken at the Ball Aerospace facility and all 3 (4) photographs seem to match that facility with both staff and ceiling details.

The smaller colored lines indicate multiple refractions or reflections of the ceiling grid.

The orange lines indicate the angle of the secondary reflections inside the facility.

The red lines shows the angles where the chips do not have AR and do not have the sapphire lens on top, so no curvature and no ceiling reflection at their angle, but do reflect the facility as the secondary reflection in the CCD's.

The curvature of both the grid reflected at least twice and of the secondary reflection in each block is due to the curvature of the sapphire lens that covers each CCD pair as shown in the diagram.

Reason for only one side of each pair showing the features is that each pair are placed opposing eachother.

Hope thats on the right track..

enter image description here

  • $\begingroup$ Still off track. AR becomes colored at increasing angle, w/o cut-off. You ignore the stripes changing orientation with the tiles, see my EDIT 2. $\endgroup$
    – Rainald62
    Feb 24 at 20:51

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